Update framework testing infrastructure and standards

- Add phpt test runner and suite for app functions
- Introduce testing workflow to AGENT.md
- Add tests for cache, content, context, helpers, hooks, plugins,
  rendering
- Mount tests directory in dev container

devel/tests/rendering/render_content_file.phpt

@@ -0,0 +1,39 @@
+--TEST--
+renderContentFile: renders .html by inclusion and .md via Parsedown
+--FILE--
+<?php
+require '/var/www/app/context.php';
+require '/var/www/app/hooks.php';
+require '/var/www/app/rendering.php';
+
+$dir = sys_get_temp_dir() . '/phpt_render_' . getmypid();
+mkdir($dir);
+
+// HTML file — returned as-is (direct inclusion)
+$htmlFile = "$dir/page.html";
+file_put_contents($htmlFile, '<p>Hello <strong>world</strong></p>');
+echo trim(renderContentFile($htmlFile)) . "\n";
+
+// Markdown — converted to HTML by Parsedown
+$mdFile = "$dir/post.md";
+file_put_contents($mdFile, "# My Title\n\nA paragraph.");
+$html = renderContentFile($mdFile);
+echo (str_contains($html, '<h1>My Title</h1>') ? 'h1 ok' : 'h1 missing') . "\n";
+echo (str_contains($html, '<p>A paragraph.</p>') ? 'p ok' : 'p missing') . "\n";
+
+// Unknown extension — returns empty string
+$txtFile = "$dir/notes.txt";
+file_put_contents($txtFile, 'raw text');
+echo (renderContentFile($txtFile) === '') ? 'empty ok' : 'not empty';
+echo "\n";
+
+unlink($htmlFile);
+unlink($mdFile);
+unlink($txtFile);
+rmdir($dir);
+?>
+--EXPECT--
+<p>Hello <strong>world</strong></p>
+h1 ok
+p ok
+empty ok